1. Field of the Invention
The present invention is related to a constant current source device and a method for manufacturing the constant current source device.
2. Description of the Related Art
“MOSFET” is an abbreviation of “metal-oxide-semiconductor field effect transistor” whose theorem is the foundation of modem IC (integration circuit) chips. A depletion mode MOSFET device comprises three basic portions: a source (S), a gate (G) and a drain (D). An N-channel depletion mode MOSFET has a shallow doped layer located close to the surface of the gate between the source and drain and with the same polarity as the source and drain. While the gate to source voltages are positive, the saturation conduction currents will increase as the gate to source voltages are raised. This character is the same as an enhancement mode MOSFET. However, while the gate and the source are at the same potential and the drain is applied with a positive bias, the depletion transistor may show passing through the rapid ascending linear region and then going into the constant current saturation region. The drain voltage at this point is called a saturation voltage. The drain saturated current level is related to the concentration and the depth of the shallow layer of the doped layer. Generally, the higher the concentration and the deeper the depths are, the larger the current will be. While the gate and the source are applied with a negative bias, the channel can be cut off, and the conduction current is zero. The gate voltage at this point is defined as a threshold voltage. However, if the concentration of the channel is too dense or the depth is too deep, the gate will be unable to cut off the channel current. While the voltage between the gate and the source is zero in the depletion mode MOSFET, the drain-source current has already shown to be constant current conductively. This character is inconvenient in the logic application when comparing with an enhancement mode MOSFET. Thus, the depletion mode MOSFET has not yet been used as an independent device in industry in the past. Since the depletion mode MOSFET has a characteristic of conduction while the gate voltage is zero and as the drain voltage is increased, the current basically stays in the saturation area till the drain breakdown, so it is suitable for the constant current source. For example, if the range of working voltage in the depletion MOSFET can be raised, that is, the breakdown voltage of the drain is raised to over 50 volts, the depletion mode MOSFET can be applied extensively as a constant current source with over voltage protection for the DC load which is connected directly with the AC power after regulating and filtering. If the drain's breakdown voltage of the depletion MOSFET is under 15 volts, the depletion MOSFET still can be used as a low voltage constant current source in the DC power, such as a constant current source for LED miners lamps.
In the practical application, although there is no need to have large power consumption for many loads, it often requires the electrical power maintaining the voltages and currents relatively stable within a certain range. At the same time, it requires certain over voltage and current protection functions for some critical elements within the loads. To solve these kinds of electricity supply issues within the load, a constant current or voltage stabilization power is normally adopted, and the over current protection circuit still needs to be added in the power circuit for some elements of the load under abnormal conditions. Thus, this will result in more power elements being required, in complex circuits, and in large proportion of wasted power under normal operation.
Currently, the applications for LEDs are becoming more and more popular, and LED lamps used for the indoor and outdoor lightening also are becoming more and more extensive. However, the current LED driving circuits all need to include the current stabilization circuit, which requires more circumferential components. Even though the luminance of the LED has better stability and smaller lighting brightness variation, the cost of the circumferential components is very high. Furthermore, the power consumption for the circumferential components has occupied 20-30% of the total power such that the advantages of energy conservation and power saving of the LED can not be fully elaborated. If a constant current source device is set to serially connect with the LED, it is able to solve the above problems. However, there currently is no such kind of independent constant current source device.